Željava Air Base is an underground military aviation complex built by the former Socialist Federal Republic of Yugoslavia. It is the largest in the Balkans and one of the most extensive underground air bases in Europe. Known by the code name "Objekt 505," the base was constructed inside the Plješevica Mountain near the town of Bihać, on the border between Bosnia and Herzegovina and Croatia. Thanks to its structure protected against nuclear attacks and its strategic location, the base was built as one of the key military facilities of the Cold War era.

Historical Background

The construction of Željava Air Base began in 1957 and was completed in 1968. It is estimated that a total budget equivalent to approximately 6 billion U.S. dollars (in today's value) was spent on the project. The base was built as part of Yugoslav President Josip Broz Tito’s strategic defense vision. Pursuing a policy of balance between NATO and the Warsaw Pact, Yugoslavia invested in such underground bases to defend the country against potential attacks.

During the 1990s, following the breakup of Yugoslavia and during the Bosnian War, the facility was rendered inoperable by the Yugoslav People’s Army (JNA) in 1992. Explosives were used to destroy it in order to prevent the strategic equipment inside from falling into enemy hands.

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Location and Structural Features

Željava Air Base is located between the Lika region of Croatia and the Una-Sana Canton of Bosnia and Herzegovina. The base was constructed by carving into the Plješevica Mountain and is supported by an extensive system of mountain tunnels and internal cave routes.

• Underground Hangars: The base features five main runways and connecting taxiways, linked through a 3.5-kilometer-long tunnel system that runs through the mountain.
• Housing Capacity: It is spacious enough to accommodate more than 120 military aircraft (primarily MiG-21s) underground.
• Climate Control: Filtration systems were in place to protect against nuclear, biological, and chemical attacks.
• Secrecy and Protection: Being embedded within the mountain, it was shielded from radar detection.
• Personnel Areas: Facilities included a mess hall, dormitories, command center, communication infrastructure, and emergency systems.
• Runways: There were four concrete runways, each 2,500 to 3,000 meters in length. One of the runways had a direct connection to the underground tunnel system.

Military Function and Strategic Role

The base was operated by the 352nd Fighter Aviation Regiment of the Yugoslav Air Force. Especially during the Cold War, Yugoslavia, which pursued a policy of neutrality between NATO and the Soviet Union, developed its defense strategies from this base against potential attacks from either side. Thanks to its radar systems, all aerial movements from the Adriatic coast could be monitored, and if necessary, air superiority across the Balkans could be coordinated from here.

Additionally, active radar surveillance towers—such as the Celopek radar station—were located around the base and operated in direct integration with Željava.

Destruction and Abandonment

In 1992, during the height of the civil war, the base was under the control of the Yugoslav military. As forces withdrew, the facility was destroyed using more than 30 tons of explosives. During this demolition, the underground galleries, runway connections, and technical systems were rendered completely inoperable. Today, the interior of the base remains hazardous, with areas of collapse, remnants of munitions, and mined zones. Therefore, visits outside the controlled areas are prohibited.

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Current Status and Tourism Potential

Although Željava Air Base is officially abandoned today, it has become a point of interest due to its historical and architectural significance. Since the 2000s, it has attracted adventure tourists, photographers, military history enthusiasts, and documentary crews. The Croatian Ministry of Tourism and local authorities have considered reorganizing the site under the framework of “military tourism” and opening its safe sections to visitors in a controlled manner. The surrounding forested areas are suitable for camping and hiking activities. Additionally, remnants of abandoned MiG-21s can be seen in the area known locally as the “aircraft graveyard.”